Hello Yuno! Thanks for checking out this game!
In order to start Anthony route, you need to visit the courtyard before the attack on the castle. Taking a walk you will find some vines growing in the wall. Then when you get shut in the wardrobe, pick either the option to peek or escape and leave the bedroom.
You will have the option to go to the courtyard and try to climb the vines to scale the wall to escape the castle. Doing so will run into Anthony.
To start Simon route you need to either escape from the wardrobe or peek and leave the bedroom.
You will have the option to look at the battle unfolding at the front gate. If you go through the gate while Isaac has pushed the enemy back, you can observe him and have the opportunity to make a run for it while he's distracted. Doing so you will encounter Simon.
Hope this helps! Best wishes <3